Non-chokable pumps are often used for waste waters charged with solids. These non-chokable pumps have maximum efficiencies of approximately 50%, but in practical application the efficiency is below 40%. These low efficiencies result in an overdimensioning of the driving motor. Since in submersible motor-driven pumps the motor represents approximately 80% of the value of a pump, there is an increase in the investment costs required to deliver a predetermined quantity of liquid to a given height.
Non-chokable pumps also have the peculiarity that solids accumulate at the center of the rotor and reduce the unobstructed flow passage, so that the pump output is throttled. The pump itself does not become clogged, since the end face of the rotor is well away from the wall of the pump casing.
Rotors having an improved efficiency are single-channel rotors having only one blade. These rotors attain efficiencies of over 70% if they are spatially curved. Because of the spatial curvature, the rotors cannot be turned down to reach other operating points. A separate rotor is therefore required for each characteristic line and also its own model in the foundry. Cylindrical rotors can be turned down and have a slightly lower efficiency than the spatially curved ones.